[llvm] [LV] Support argmin/argmax with strict predicates. (PR #170223)

[via llvm-commits]

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@@ -1454,12 +1461,7 @@ static bool handleFirstArgMinOrMax(
          "FindIV reduction must not be scaled");
   VPValue *MaxIV =
       Plan.getConstantInt(APInt::getMaxValue(Ty->getIntegerBitWidth()));
-  ReductionStyle Style = RdxUnordered{1};
-  auto *FirstIdxPhiR = new VPReductionPHIRecipe(
-      dyn_cast_or_null<PHINode>(FindLastIVPhiR->getUnderlyingValue()),
-      RecurKind::UMin, *MaxIV, *FindIVSelectR, Style,
-      FindLastIVPhiR->hasUsesOutsideReductionChain());
-  FirstIdxPhiR->insertBefore(FindLastIVPhiR);
+  FindLastIVPhiR->setOperand(0, MaxIV);
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ayalz wrote:

Re: using poison instead of MaxUInt - a `select` should prevent poison from propagating? If overall min/max was set in some iteration, then all lanes resulting in other min/max should have their index replaced by MaxUInt before reducing, including all poison indices - whose resulting min/max is the initial extremal value. Otherwise, all indices are poison, and its reduced result (poison?) should be replaced by select'ing the original default index instead?

https://github.com/llvm/llvm-project/pull/170223